The present invention relates to suspensions for single degree of freedom gyroscopes and more particularly to apparatus for increasing suspension stiffness affecting rotational deviation or displacement about the input axis of such a gyro and utilization of the existing suspension hardware to provide a means of readout of any residual angular misalignment.
Single degree of freedom gyros are used to control and stabilize platforms to point optical and other devices through extremely great distances in tracking and pointing applications. It is essential that the rate signals emanating from these instruments be extremely steady as any "jitter" will result in a blurring effect. This gyro error phenomenon is referred to generally as noise. In addition to the stabilizing function, another important requirement for this type of instrument is that it must be capable of being commanded to rotate to different positions at fairly high angular rates, and when the new position has been reached, the time to return to equilibrium (for a constant pointing signal) must be minimal.
Many single degree of freedom gyros include a gyro wheel mounted within a "float" which floats in a viscous fluid inside an outer case. The more sophisticated single degree of freedom gyros employ electromagnetic suspensions to position the float within the case of the gyro without actual physical contact. The operation of such a suspension is described in U.S. Ser. No. 617,228 filed June 4, 1984, and now U.S. Pat. No. 4,648,284 issued Mar. 10, 1987, and assigned to the same entity as the present application, and its teachings are incorporated herein by reference. The invention of that application has to do with suspension forces along the output axis of the gyro. In U.S. Pat. No. 4,648,284 during instrument warm up or turn on, the float tends to move axially in a direction along the output axis of the gyro. The invention of U.S. Pat. No. 4,648,284 is thus concerned in utilizing an active mode in more rapidly restoring the float to an equilibrium position along the output axis.
As will also be appreciated by those skilled in the art, the float in a single degree of freedom gyro during maneuvers tends to rotate or be displaced about the input axis IA. Consequently, true angle indication after such displacement is not possible until the suspension again reaches its equilibrium (centered) position about the input axis. It is thus highly desirable to employ suspensions having maximum stiffness with respect to movement or displacement about the input axis. To the extent the float has not returned to its equilibrium position after a maneuver, a proportionate pointing angle error exists. Also, the greater amount of time during which the float is returning to its equilibrium position, the greater amount of time is consumed before which pointing accuracy can be achieved. The heavy viscous damping of the fluid in which the float resides is a factor which resists rapid recentering. Increasing suspension stiffness thus will decrease the amount of time consumed in restoration to the equilibrium or centered position.
Increasing suspension stiffness will also increase the maximum slew rate of a single degree of freedom gyro. Frequently the maximum rate within which a gyro can perform is limited by the suspension "bottoming out" with respect to the output axis against its radial stops. Increasing suspension stiffness which results in higher slew rates will have application during gyro compassing. One method of gyro compassing involves rapid slew about the output axis of the gyro through 180.degree.. In order to gyrocompass by case rotation, the input axis pointing direction is redirected 180 degrees. This is accomplished by rotating about either the spin axis or about the output axis. Because of the gyroscopic effect, larger torques are involved when rotating about OA than when rotating about SA. Despite this disadvantage of the greater torque, rotation is usually accomplished by rotating about a vertical output axis for the reason that drift uncertainty is superior in this attitude. Thus performance (response time) is improved by a stiffer suspension.
Another problem area of single degree of freedom gyros results when there is an unrecognized angular displacement about the input axis between the float and the case. This is the small angle that remains when the float is near its equilibrium position. It is thus desirable to have precise knowledge of the extent to which the float is not in its equilibrium, or precise pointing position.
It is therefore an object of the present invention to increase suspension stiffness with respect to movement or displacement about the input axis of a gyro.
It is another object of the invention to increase suspension stiffness in a gyro by making a direct measurement of float radial position by utilizing information provided by the suspension currents.
It is yet another object of the invention to provide an increase in suspension stiffness which will increase the slew rate capability of the gyro.
A still further object of the invention is to derive accurate angle information for any residual float angle deviation from the equilibrium position in a gyro to provide a precise indication of error angle.